Cardiac arrest is a significant public health problem cutting across age, race, and gender. A positive impact on cardiac arrest survival has been demonstrated with the substantial reduction in time to defibrillation provided by the widespread deployment of automated external defibrillators (AEDs), and the use of implantable cardioverter defibrillators (ICDs) and implantable pulse generators (IPGs). Examples of AEDs are described in U.S. Pat. Nos. 5,607,454, 5,700,281 and 6,577,102; examples of ICDs are described in U.S. Pat. Nos. 5,391,186, 7,383,085, and 4,407,288, and examples of IPGs are described in U.S. Pat. Nos. 4,463,760, 3,978,865, and 4,301,804, the disclosures of which are incorporated by reference herein.
Optimal resuscitation therapy for out of hospital (OOH) cardiac arrest is the subject of substantial ongoing research. Research has been clear in demonstrating that the timing of resuscitation is of critical importance. For example, there is less than a 10% chance of recovery just ten minutes after the onset of ventricular fibrillation (VF). This knowledge led to the recent widespread deployment of AEDs, primarily in public areas with a high population concentration such as airports and shopping malls. A positive impact on cardiac arrest survival has been demonstrated due to the substantial reduction in time to defibrillation as a result of more available access to AEDs. In addition, for those patients identified as being at particularly high risk, an implantable cardioverter-defibrillator is often implanted in order to address episodes of cardiac arrest without the involvement of a rescuer.
In the case of VF, performing CPR-type chest compressions before defibrillation and minimizing the time to defibrillation shock following the cessation of the CPR chest compressions is important in facilitating effective recovery especially in cases of long duration VF. It is generally believed that perfusion of the myocardium achieved during CPR preconditions the heart for the defibrillating shock. Despite the importance of CPR, it is often not performed in the field for a variety of reasons.
Cardiac electrotherapy stimuli having an amplitude that is greater than that of pacing-type stimuli, but less than the amplitude and energy level associated with defibrillation-type stimuli, are known in the art as medium voltage therapy (MVT). For example, U.S. Pat. No. 5,314,448 describes delivering low-energy pre-treatment pulses followed by high-energy defibrillation pulses, utilizing a common set of electrodes for both types of stimuli. According to one therapeutic mechanism of this pre-treatment, the MVT pulses re-organize the electrical activity within the cardiac cells of the patient to facilitate a greater probability of successful defibrillation with a follow-on defibrillation pulse. U.S. Pat. No. 6,760,621 describes the use of MVT as pretreatment to defibrillation that is directed to reducing the likelihood of pulseless electrical activity and electromechanical dissociation conditions as a result of the defibrillation treatment. The mechanism by which these results are achieved by MVT has been described as a form of sympathetic stimulation of the heart. These approaches are directed to influencing the electrochemical dynamics or responsiveness of the heart tissues.
MVT has also been recognized as a way of forcing some amount of cardiac output by electrically stimulating the heart directly with stimuli that cause the heart and skeletal muscles to expand and contract in a controlled manner. See U.S. Pat. Nos. 5,735,876, 5,782,883 and 5,871,510. These patents describe implantable devices having combined defibrillation, and MVT capability for forcing cardiac output. U.S. Pat. No. 6,314,319 describes internal and external systems and associated methods of utilizing MVT to achieve a hemodynamic effect in the heart as part of an implantable cardioverter defibrillator (ICD) for purposes of achieving a smaller prophylactic device. The approach described in the '319 patent uses the MVT therapy to provide a smaller and less expensive implantable device that can maintain some cardiac output without necessarily providing defibrillation therapy.
Unlike a conventional defibrillator or an IPG, which operates with the primary purpose of restoring a normal cardiac rhythm, MVT stimulation can be used to provide cardiac output, which in turn causes perfusion to the heart and brain, as well as other critical body tissues. By providing perfusion to the heart and other vital organs, MVT prolongs the life of the patient even while the patient continues experiencing the arrhythmia. Additionally, MVT improves the likelihood of successful defibrillation or of a spontaneous return of circulation. In another application, MVT may be utilized to place a heart into a distended state by continuing venous return in the absence of cardiac output, thus making it more likely to return to a spontaneous pulsatile rhythm. An AED equipped with MVT can provide consistent high quality chest compressions. In the case of an implanted ICD or IPG, back up chest compressions provided by MVT can, in one sense, be even more important than in an external, since in the case of the implantable device there may be no rescuer available to perform CPR when needed.
Recent studies have identified an increasing incidence of patients whose initial rhythm is not VF, but may be (PEA), or asystole. In addition in many cases an unsuccessful defibrillation shock (whether from an AED or an ICD) results in PEA, asystole or persistent VF. In all these cases the indicated therapy is CPR type chest compressions. Conventional ICD, IPG, and AED devices, even those enabled with MVT, work very well to treat VF, but provide little or no therapy for other common arrhythmias of cardiac arrest, namely, pulseless electrical activity (PEA) and asystole.
While developments in defibrillator technology, both automatic external defibrillators (AEDs) and implantable cardioverter defibrillators (ICDs) have made great strides in aiding the electrical cardiac resuscitation of individuals experiencing cardiac arrest, a need exists for a solution that can effectively treat the increasing number of victims that either present with non-VF cardiac arrest or are shocked into a non-VF non-pulsatile rhythm such as PEA or asystole.
U.S. Patent Application Publication No. 2006/0142809, currently pending, describes a technique and associated apparatus that combines defibrillation therapy with MVT into an external device having a capability to perform electrical CPR. Externally-applied MVT is proposed for stimulating skeletal and sympathetic muscles in addition to myocardial muscle tissue to effect chest compression and even ventilation in the patient. The '809 publication reflects the knowledge in the art that due to the inclusion of differing time constant components in an MVT waveform, the waveform can stimulate contraction of a variety of different types of muscles, e.g., myocardial, skeletal, sympathetic muscles, and the phrenic nerve. Varying and controlling the MVT waveform parameters, including variation of the musculature targeted by the waveform, is described as a way to maximize coronary perfusion pressure generated by application of MVT.
Notwithstanding the advancements in MVT for cardiac output forcing made to-date, known MVT techniques have been shown to be effective for only a limited time due to muscle fatigue resulting from application of the MVT. Particularly, after repeated application of the MVT electrical pulses, the muscles being stimulated become unresponsive to further MVT stimulation, resulting in a drop-off in coronary perfusion. A solution is therefore needed for enabling longer duration and more productive MVT sessions.